To overcome this major disadvantage and to obtain a non-stick coating whose mechanical properties are reinforced, it is known to the person skilled in the art to form a multilayer non-stick coating whose first layer, starting from the bottom of the utensil (commonly known as the “primer layer”) and acting as binding layer for one or more upper layers of the non-stick coating (commonly called the “top layers”), in addition to the sintered fluorocarbon resin, contains a high quantity of mineral or hard organic fillers (for example, silica, quartz or aluminium).
However, this type of reinforcement necessarily remains limited since the filler content in the primer layer cannot exceed a few percent by weight of the total weight of the primer layer. Over and above a certain threshold quantity of filler, typically over and above 15% by weight of filler in the primer layer, this layer may lose its cohesion.
In addition, it is also known to the person skilled in the art to form a hard sub-layer or a hard base between the substrate (in this case, the inner surface of a cooking utensil) and the non-stick coating (notably the primer layer).
The forming of a hard sub-layer or hard base between the substrate and the non-stick coating not only allows the non-stick coating to be mechanically reinforced (notably in terms of hardness) but it also imparts high scratch resistance.
The sub-layer or hard base forms a barrier which prevents scratches from reaching the surface of the substrate.
Hard sub-layers are known formed by a polymer such as polyamide imide (PAI) and/or oxy-1,4-phenylene-oxy-1,4 phenylene-carbonyl-1,4-phenylene (PEEK) as taught in international applications WO 00/54895 and WO 00/54896 on behalf of the applicant.
Also, hard bases in metal or alumina are known. If the hard base is formed of alumina, it can be deposited by heat spraying onto the substrate here consisting of the inner surface of the bottom of the utensil. In the particular case of a substrate in aluminium or aluminium alloy, a hard base in alumina can also be formed directly on the substrate by anode oxidation thereof. For example, European patent EP 0 902 105 describes a non-stick coating deposited on said hard base which has high scratch and abrasion resistance. However, this non-stick coating has the disadvantage of not being sufficiently resistant to corrosion or to separation (cleavage). In addition, the forming of alumina, in particular using the so-called hard anodization process, is very costly from an energy viewpoint since high electric powers must be provided to maintain low temperatures in the anodizing baths.
A hard metallic base is also known which is formed on the inner surface of a cooking utensil to reinforce the non-stick coating layer. For example, U.S. Pat. No. 5,455,102 describes a cooking utensil comprising a metal body sheet whose inner roughened surface, starting from the bottom of the body sheet, is successively coated with a hard metal layer (notably in steel, copper or aluminium) which is scratch-resistant and a lubricating layer essentially consisting of PTFE. The hard scratch-resistant layer has a thickness of between 40 and 90 μm with a mean arithmetic roughness (Ra) of between 5 μm and 8 μm. The lubricating layer in PTFE is applied using known spray techniques, while the hard scratch-resistant layer is formed by electric-arc spraying. The forming of said hard layer has the disadvantage, however, of requiring the use of costly depositing equipment (electric arc) which is also energy-consuming and little productive. The use of a different metal is also a source of new difficulties by creating an electrolytic couple which generates weakening of corrosion resistance.